AN: I don't own 40K or any associated products. I did, however, create the original characters you will see.
On the hiveworld of Celibae Secundus, the forces of Chaos were hard at work.
Dark priests, hidden deep in the underhives of the massive, continent spanning cities, brought about their ill intentions as they cast their eyes into the pool of blood. The crimson surface shimmered, spiraling and becoming a whirlpool in the shallow basin they had dug into the stone. One of their number began to panic as voices heard only to him began to cry out his name. Hands reached out of the portal, long and gaunt and ending in claws that gouged the stone as something inhuman tried to claw its way out of the portal, only to be ripped away from their purchase in the Materium and pulled back into the portal with a ghastly cry.
And then a figure rose from the swirling pool, standing higher than any of the men in the cavernous room. He was clad in armor, a matte grey with golden trim, and a number of horns wrapped around his helm from a strange sigil on his brow. Profane runes shimmered brightly in the metal, almost glowing with an ethereal light. He was armed just as heavily as he was armored, an Ætherflame Cannon on his spine and a strange sonic pistol on his hip with a mighty staff in one hand and a long dagger on his other hip. The mortals around the pool saw him rise, and immediately knelt as the portal closed beneath the fallen Astartes. Argorathrax inhaled deeply, the chemical stench of the hive world's retched, ultra-toxic waste burning into his suit's air filters. He eyed the frail and desperate men around him, smiling at their obvious fear and awe. It was only right, the ancient sorcerer decided, that they be given the courtesy of a thank you, for they had inadvertently freed him from one of Tzeentch's punishments. To be forced to wander endlessly in a desert filled with predators both natural and un, with only the accursed former Sister he once called apprentice for company. How Argorathrax wished he could just throttle her, make her beautiful face run purple before her head exploded in his hands like-
"By the power of Tzeentch, I bind you, daemon!"
The Astartes felt the clamp of magic reach for him, breaking him from his musings. He reached to it with his mind, and followed the link back to a man who stood differently from the others, an opulently robed figure in gold and saffron robes that regarded him with an arrogant and disdainful eye.
Argorothrax tilted his head, and the man flew away as telekinetic power wrapped around him and bodily threw him to the ceiling. To bind him, the sorcerer known once as Anu'thoth of the Raptora Cult? The fallen Son of Magnus chuckled as he turned to the next man.
"You have summoned me here for a purpose. I am called Argorathrax, son of Magnus. What is your will?"
Silence reigned, before one of the other men stepped forward and spoke confidently.
"High King Tiranath has turned from the light of the God-Emperor, slaughtering our people in the name of sport. He has turned the city above into a war zone, and the Imperium has forsaken us in turn. I, WE, we beg of you, save us!"
Argorathrax was silent as he stared at the man, his dusty flesh crawling over itself inside of his armor. He reached out with his mind, casting his consciousness to the surface world. The hive cities had fallen to wanton bloodshed, and a civil war raged rampantly above his head. He chuckled, and faced the man who spoke.
"Very well. I shall summon my own cabal to aid me in this venture."
The men nodded vigorously, relieved at his acceptance. The priestly man that had been thrown to the ceiling then fell, Argorathrax mentally throwing his perforated body into the pool of blood as he began to speak the Black Tongue. The veil of reality shifted, the ground writhing under his feet as he pulled the life-giving energy of the planet from it. The man once known as Anu'thoth rose his staff, and smote the ground. The cavern wall nearest to him split open, reality sundering at his will. From the crack in both stone and space, a band of humans and daemons alike strode out, led by a woman in revealing armor that clutched what looked to have been a full person strewn over a massive cannon. Her purple eyes were glinting with desire as she bit her lip, the inhuman beauty staring at the man who'd called her here. Beside her stalked a snarling man with black eyes, as large as Argorathrax in his own armor, with brass arms and armor as red as blood over his chest and legs. The final touch was a thin woman, frail and haggard looking. Her beauty was faded, lank black locks falling over her glassy green eyes as she smiled a motherly smile at the sorcerer as he spoke.
"Thaena, Karagôr, Ithra. Welcome to Celibae Secundus. I do believe it is the perfect place to host this grand gathering."
The sickly woman coughed violently, and spoke, her voice hoarse and unused.
"It will be a lovely place to spread Father's love..."
She doubled over and vomited, a bright green goo that quickly rose into the diseased form of a Nurgling. Karagôr eyes her with disgust before he spoke, thick Terran accent, reminding Argorathrax of the World Eater's brother, Khârn, rumbling as he did. Contempt layered his words, even as he glared at his cousin.
"You haf mentioned dis gadering before... Must it include dese two?"
Argorathrax chuckled and shrugged at his gene-cousin, and the woman known as Ithra cooed in the background to her beloved familiar, uncaring of the boils and lesions dancing across her flesh as she nuzzled the little beast and let it lick her face.
The Slaaneshi former Sister of Battle spoke next, raising her sonic cannon up onto her shoulder with ease and grinning.
"It's wonderful to see you again, master. You were so creative, leaving me to be gnawed on by that tree whilst you came here..."
Argorathrax quietly wished it had finished the job, but diligently ignored his apprentice. Behind them, an army of the shambling corpses of all manner of beings and lumbering beasts of Nurgle, courtesy of Ithra and her penchant for the necromantic arts, began to pour into real space. The living still near the quartet fled into their homes as the massive beasts began to rampage with the intention of playing, bringing unspeakable diseases with them and forever tainting the ground they trampled over.
"There is plenty of excitement on the surface. I'm sure you can find some manner of enjoying yourselves. Rest for now, prepare yourselves. We will attack the surface in an hour, to take it for Chaos."
The trio of newcomers laughed, and went their separate ways to bring Chaos to this world...